.\" $OpenBSD: crypt.3,v 1.5 1996/12/10 09:06:09 deraadt Exp $
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.\" FreeSec: libcrypt
.\"
.\" Copyright (c) 1994 David Burren
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.\" Manual page, using -mandoc macros
.\"
.Dd March 9, 1994
.Dt CRYPT 3
.Os "FreeSec 1.0"
.Sh NAME
.Nm crypt ,
.Nm encrypt ,
.Nm setkey
.Nd DES encryption
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft char *
.Fo crypt
.Fa "const char *key"
.Fa "const char *salt"
.Fc
.Ft void
.Fo encrypt
.Fa "char *block"
.Fa "int edflag"
.Fc
.Fd #include <stdlib.h>
.Ft void
.Fo setkey
.Fa "const char *key"
.Fc
.Sh DESCRIPTION
The
.Fn crypt
function performs password encryption, based on the
.Tn NBS
Data Encryption Standard (DES).
Additional code has been added to deter key search attempts.
The first argument to
.Fn crypt
is a
.Dv null Ns -terminated
string, typically a user's typed password.
The second is in one of two forms:
if it begins with an underscore (``_''), an extended format is used
in interpreting both the key and the salt value, as outlined below.
.Ss Extended crypt:
.Pp
The
.Ar key
is divided into groups of 8 characters (the last group is null-padded)
and the low-order 7 bits of each each character (56 bits per group) are
used to form the DES key as follows:
the first group of 56 bits becomes the initial DES key.
For each additional group, the XOR of the encryption of the current DES
key with itself and the group bits becomes the next DES key.
.Pp
The
.Ar salt
is a 9-character array consisting of an underscore, followed
by 4 bytes of iteration count and 4 bytes of salt.
These are encoded as printable characters, 6 bits per character,
least significant character first.
The values 0 to 63 are encoded as ``./0-9A-Za-z''.
This allows 24 bits for both
.Fa count
and
.Fa salt .
.Ss "Traditional" crypt:
.Pp
The first 8 bytes of the key are null-padded, and the low-order 7 bits of
each character is used to form the 56-bit
.Tn DES
key.
.Pp
The
.Fa salt
is a 2-character array of the ASCII-encoded salt.
Thus, only 12 bits of salt are used.
.Fa count
is set to 25.
.Ss Algorithm:
.Pp
The
.Fa salt
introduces disorder in the
.Tn DES
algorithm in one of 16777216 or 4096 possible ways
(ie. with 24 or 12 bits: if bit
.Em i
of the
.Ar salt
is set, then bits
.Em i
and
.Em i+24
are swapped in the
.Tn DES
E-box output).
.Pp
The DES key is used to encrypt a 64-bit constant, using
.Ar count
iterations of
.Tn DES .
The value returned is a
.Dv null Ns -terminated
string, 20 or 13 bytes (plus null) in length, consisting of the
.Ar salt ,
followed by the encoded 64-bit encryption.
.Pp
The functions,
.Fn encrypt
and
.Fn setkey
provide access to the
.Tn DES
algorithm itself.
.Fn setkey
is passed a 64-byte array of binary values (numeric 0 or 1).
A 56-bit key is extracted from this array by dividing the
array into groups of 8 and ignoring the last bit in each group.
That bit is reserved for a byte parity check by DES, but is ignored
by these functions.
.Pp
The
.Fa block
argument to
.Fn encrypt
is also a 64-byte array of binary values.
If the value of
.Fa edflag
is 0,
.Fa block
is encrypted; otherwise, it is decrypted.
The result is returned in the original array
.Fa block ,
after using the key specified by
.Fn setkey
to process it.
.Pp
The function
.Fn crypt
returns a pointer to the encrypted value on success, and NULL on failure.
.Pp
The
.Fn crypt
and
.Fn setkey
functions all manipulate the same key space.
.Sh SEE ALSO
.Xr login 1 ,
.Xr passwd 1 ,
.Xr getpass 3 ,
.Xr compat 5 ,
.Xr passwd 5
.Sh LEGACY SYNOPSIS
.Fd #include <unistd.h>
.Pp
.Ft int
.br
.Fo encrypt
.Fa "char *block"
.Fa "int edflag"
.Fc ;
.Pp
The function
.Fn encrypt
returns 0 on success and 1 on failure.
.Pp
.Ft void
.br
.Fo setkey
.Fa "const char *key"
.Fc ;
.Pp
The include file
.In unistd.h
is necessary and sufficient for the
.Fn setkey
function.
.Sh BUGS
The
.Fn crypt
function returns a pointer to static data, and subsequent calls to
.Fn crypt
will modify the same object.
.Sh HISTORY
A rotor-based
.Fn crypt
function appeared in
.At v6 .
The current style
.Fn crypt
first appeared in
.At v7 .
.Pp
This library (FreeSec 1.0) was developed outside the United States of America
as an unencumbered replacement for the U.S.-only libcrypt encryption
library.
Programs linked against the 
.Fn crypt
interface may be exported from the U.S.A. only if they use 
.Fn crypt
solely for authentication purposes and avoid use of
the other programmer interfaces listed above. Special care has been taken
in the library so that programs which only use the 
.Fn crypt
interface do not pull in the other components.
.Sh AUTHOR
David Burren <davidb@werj.com.au>
